The typical methods of making nanoparticles in liquids involve either a synthetic chemical process or a physical method. In chemical processes nanoparticles are created using chemical synthesis under very controlled conditions. In the basic process the reactants are reacted in a liquid solvent and the desired nanoparticles are precipitated out of the liquid solvent. Often this method requires use of surface controlling agents to prevent excessive growth of the particles and agglomeration. Use of the surface controlling agents helps to control particle size by giving the particles an electric charge thereby reducing growth and agglomeration. Because the chemical synthesis is generally conducted at low temperatures in slow and well controlled reactions, particle size control can be achieved precisely. The drawback of chemical processes is that many surface control agents used in the synthesis are disadvantageous to the eventual end use of the nanoparticles. For example, for applications in biochemical sensors, the surface control agents contained in the colloids can introduce complex background noise peaks, complicating identification of the subject chemical and reducing the sensors's sensitivity.
In physical methods of producing nanoparticles the source materials usually start as bulk solids and are disintegrated through physical means. Common physical means include milling of a bulk solid, high voltage spark discharge, volatilization followed by condensation, and laser ablation. Particle size control can be very difficult to achieve in physical methods of nanoparticle formation. In laser ablation, material is removed from a bulk solid by vaporization due to absorption of the laser energy by the target substrate. The ability of the material to absorb the energy limits the depth of ablation. The depth of ablation is determined by the ability of the material to absorb the laser energy and the heat of vaporization of the material. The depth to which the material absorbs the energy is a function of the material's absorption coefficient, laser beam energy density, laser pulse duration, and the wavelength of the laser beam. The primary goal is to have a short pulse duration to maximize the peak power and to minimize thermal conduction to the surrounding material. For efficient removal, it is also advantageous to have a high pulse repetition rate to utilize the residual heat from the previous pulses. When wavelength variation is available, it is also desirable to choose a wavelength that has a minimal absorption depth thereby concentrating the pulse energy in the smallest volume to ensure high volatilization. Laser ablation produces a plume of ablated material comprising molecular fragments, neutral particles, free electrons, ions, and chemical reaction products. The plume can scatter the incoming laser beam and disrupt its ability to ablate additional material. Thus, the plume must always be considered in laser ablation methods. For many target materials, when pulsed laser ablation is performed in a liquid to produce nanoparticles, it has been observed that the particles remain stable against agglomeration without adding stabilizing agents such as surfactants. See for example Fumitaka Mafune, Jun-ya Kohno, Yoshihiro Takeda, and Tamotsu Kondow, Journal of Physical Chemistry B, Vol 107, pp 4218-4223, 2003. A possible reason is that the particles are charged in the plume plasma and are thus automatically stabilized due to electric repulsion.
The desired objective of nanometer particle size control is harder to achieve in physical methods such as pulsed laser ablation compared to chemical methods. This is because in laser ablation, particle generation occurs at high temperatures and within a very short time duration. This time duration is closely related to the laser pulse duration, which typically ranges from a few nanoseconds to less than one picosecond, thereby hardly allowing any intervention time for control purposes. As observed in the past, the nanoparticles produced by pulsed laser ablation often have a wide size distribution ranging from a size of a few nanometers to several hundred nanometers. Especially in the case of long laser pulse durations of greater than 1 nanosecond, the reasons for generation of large particles during ablation in a vacuum and ambient air have been extensively discussed and include violent splashing of the melt, mechanical surface damage, and explosive boiling as summarized in Li-Chyong Chen in Chapter 6 of Pulsed Laser Deposition of Thin Films, John Wiley & Sons Inc., 1994, pp 167-196. Also see more recent discussions in N. M. Bulgakova and A. V. Bulgakov in Applied Physics A, Vol 73, pp 1990208, 2001. The same physics in general applies to ablation in liquids. One consequence of a wide size distribution is a low efficiency of nanoparticle production of particles having a size of less than 100 nanometers because large particles consume most of the bulk source materials. This failure to produce a majority of particles in the less than 100 nanometer size range can be very costly for expensive source materials such as noble metals. In most applications wherein nanoparticles are used a large surface-volume ratio is preferred, which means that it is desirable that the size of the nanopartieles be less than 100 nm for most nanoparticle products. Thus, laser ablation has not found widespread use in production of nanoparticles.
Recent improvements in ultrafast laser technology have provided opportunities for real-time control of pulse parameters during pulsed laser ablation. In particular, the availability of very high pulse repetition rates of tens of MHz and above in newer pulsed lasers makes control more likely. Conventional pulsed lasers such as excimer, Q-switched Nd:YAG, Nd:glass, and Ti:sapphire all have pulse repetition rates in the range of from 10 Hz to a few kHz. The corresponding time separation between pulses is on the order of 1-100 ms for these conventional lasers. In this relatively longtime scale, each laser pulse causes an isolated ablation event, and when the next pulse arrives, the target bulk material has mostly returned to its original thermodynamic state of temperature and pressure.
If the pulse separation could be reduced to be comparable to the characteristic time scales of one or more processes during ablation, such as electric conduction, thermal conduction, stress release, and plume expansion, ablation events caused by consecutive pulses will become correlated and cumulative effects of multiple pulses could become important. A few examples of characteristic time scales of ablation-related processes include the following. The typical cooling time of a hot spot on a bulk metal surface after laser exposure is on the order of from 10 to 100 nanoseconds, for bulk insulators it is from 1 to 100 microseconds. For a focused laser beam with a Rayleigh length on the order of 100 μm, the time for a laser-induced plume to traverse this length is on the order of 100 nanoseconds in a vacuum. In ambient air or in a liquid the ablated materials will stay longer in the laser focal volume due to the ambient pressure confinement than in a vacuum thus this time is even longer in ambient air or in a liquid. This means the ablated materials will be subjected to more pulses in the same time frame. These time scales indicate that if one could achieve a laser repetition rate greater than 10 MHz, i.e. 10 million pulses per second, cumulative heating and plume-pulse interactions could become significant even for metal targets.
Previous international PCT application publication number WO/2006/030605 teaches laser ablation methods for metal nanoparticle generation in a liquid. Similar art is also described in the references Fumitaka Mafune, Jun-ya Kohno, Yoshihiro Takeda, and Tamotsu Kondow, Journal of Physical Chemistry B, Vol, 107, pp 4218-4223, 2003 and S. Besner, A. V. Kabashin, M. Meunier in Applied Physics A Vol 88, pp 269-272, 2007. In all of these prior art methods a low repetition rate pulsed laser was used. The usual pulse rate was from 10 Hz to 1 kHz. U.S. patent application Ser. No. 12/320,617 by Liu et al from the assignee of the current application teaches a pulsed laser ablation method utilizing a high repetition rate of greater than 100 kHz. However, a straightforward application of a high repetition rate pulsed laser in laser ablation as a method of producing nanoparticles has several drawbacks. When ablation is performed in a liquid solvent at a repetition rate greater than 100 kHz accumulative heating of the liquid solvent will become a problem. In addition, at a repetition rate greater than 1 MHz, the pulse energy will be limited due to limited total power of the laser. In addition to use of high repetition rate pulsed lasers in laser ablation of a target in a liquid, burst mode lasers for ablation and deposition in a vacuum chamber have been used, for example, to create nearly particle free thin films. Examples are U.S. patent application Ser. No. 12/254,076 filed on Oct. 20, 2008 and Ser. No. 12/401,967 filed on Mar. 11, 2009. In these disclosures pulsed laser deposition using ultrashort pulsed laser ablation is used to deposit substrate materials onto a substrate.
It is desirable to provide a method of ultrafast pulsed laser ablation that will lead to production of nanoparticles of a controlled size in a liquid solvent at a high rate of efficiency.